High hardness materials are widely used as coatings on various types of mechanical components and cutting tools. Such coatings impart wear and erosion resistance and thus increase the wear and erosion life of objects that have been coated. The high hardness materials can also be used to produce free standing objects which are wear resistant.
Chemical vapor deposition processes can be used to produce high hardness coatings and high hardness free standing objects. In a typical chemical vapor deposition (CVD) process the substrate to be coated is heated in a suitable chamber and then a gaseous reactant mixture is introduced into the chamber. The gaseous reactant mixture reacts at the surface of the substrate to form a coherent layer of the desired coating. By varying the gaseous reactant mixture and the CVD process parameters, various types of deposited coatings can be produced.
Deposits produced by CVD, both for coating substrates and for producing free standing objects, have suffered certain drawbacks. Although the hardness of the deposits has been satisfactory, the strength and toughness of the materials has often been lower than desired. This lack of strength and toughness is due in large part to the grain size, crystallite size, and structure of the compounds that make up the deposit. Unfortunately, regardless of tile components of the gaseous reactant mixture, typical CVD techniques produce coatings having relatively large grains which are arranged in columns. Thus, cross-sectional metallographic examination of a typical CVD deposit will show grains usually in excess of several microns in size which are arranged in columns that extend perpendicularly to the substrate surface. Such deposits are typically quite brittle since adjacent columns of grains result in long interstitial regions of weakness. Such regions are easily fractured and attacked by corrosive agents and erosive media. Because of the columnar grain structure, such deposits also have poor surface finish and poor wear and erosion resistance properties.
U.S. Pat. No. 4,162,345, issued Jul. 24, 1979 to Robert A. Holzl, discloses a method for producing deposits of tungsten and carbon or molybdenum and carbon which results in deposits characterized by a structure which is free of columnar grains and instead consists essentially of fine, equiaxial grains. These deposits have unusually high hardness and tensile strength. The Holzl patent discloses use of temperatures varying from 650.degree. C. to 1100.degree. C., which are high enough to severely degrade the mechanical properties of various carbon steels, stainless steels, nickel alloys, titanium alloys and cemented carbide.
In the method of the Holzl '345 patent, a sequence of events is made to take place which, although similar to conventional chemical vapor deposition, is not truly that. The Holzl method employs a reactor which is essentially similar to a chemical vapor deposition reactor. However, according to the Holzi method the apparatus is operated in such a manner that the typical chemical vapor deposition process does not take place. Typical chemical vapor deposition involves a single reaction by the gases in the reactor at the surface of the substrate resulting in the formation of a solid phase deposit directly on the substrate surface. On the other hand, the Holzl '345 patent describes a deposition process involving at least two distinct reaction steps. According to the Holzl method, an initial reaction is caused to take place displaced from the surface of the substrate. This reaction involves a decomposition or partial reduction of a fluoride of tungsten (preferably WF.sub.6) by a substitution reaction with an oxygen or oxygen-containing group derived from a gaseous organic compound containing hydrogen, carbon and oxygen. Subsequent reaction with hydrogen gas results in the formation of the final deposits. The material of the Holzl '345 patent is a hard metal alloy, consisting primarily of tungsten and carbon. X-ray diffraction analysis of the '345 alloy shows that the deposit is akin to tungsten but with a very finely dispersed carbide, probably in the form of WC. The Holzl patent discloses use of temperatures varying from 650.degree. C. to 1100.degree. C., which are high enough to severely degrade the mechanical properties of various carbon steels, stainless steels, nickel alloys, titanium alloys and cemented carbide.
U.S. Pat. No. 4,427,445, issued Jan. 24, 1984 to Robert A. Holzl, et al. also discloses a hard fine grained material which can be produced by thermochemical deposition, but at temperatures lower than those described in the examples of the '345 patent. Thus, where there are large differences in the thermal coefficients of expansion between the substrate material and the coating material, the '445 methodology reduces adhesion problems and problems associated with mechanical distortion, metallurgical transformation or stress relief of the substrate. The material of the '445 Holzl, et al. patent is a tungsten carbon alloy consisting primarily of a two phase mixture of substantially pure tungsten and an A15 structure.
U.S. Pat. No. 3,368,914, discloses a process for adherently depositing tungsten carbide of substantial thickness on steel and other metal substrates. The process involves first diffusing another metal on the surface of the substrate to relax the thermal expansion coefficient zone of the metal substrate. The carbide coating is then deposited on the diffused surface by CVD. The process claims it is preferable to diffuse the metal forming the carbide into the substrate. In one embodiment of the claimed process, a thin layer of W is deposited on the metal surface using 600.degree.-1000.degree. C. temperature. After coating W, the temperature is increased to approximately 1000.degree.-1200.degree. C. and held there for a significant period of time to permit diffusion of W into the metal. The diffused surface is then coated with tungsten carbide using WF.sub.6, CO and H.sub.2. In the alternative embodiment, a pack diffusion technique is used for achieving diffusion of W into metal. Temperature ranging from 1000.degree.-1200.degree. C. is used in the pack diffusion step. The diffused metal surface is then coated with tungsten carbide.
U.S. Pat. No. 3,389,977, discloses a method of depositing substantially pure tungsten carbide in the form of W.sub.2 C, free from any metal phase. Pure W.sub.2 C is deposited on a substrate by reacting WF.sub.6 and CO. The substrate is heated to a temperature in excess of 400.degree. C. The adherence of W.sub.2 C to steel is improved by first cleaning the surface and then depositing with a thin film of W followed by W.sub.2 C using a temperature ranging from 600.degree.-1000.degree. C. Since initial deposition of tungsten is lo conducted at or above 600.degree. C., the '977 process is not feasible for providing erosion and wear resistance coating on various carbon steels, stainless steels, nickel and titanium alloys without severely degrading their mechanical properties. Additionally pure W.sub.2 C deposited according to the teachings of the '977 patent consists of columnar grains. The '977 patent does not describe a process for depositing tungsten carbide coating having a non-columnar microstructure.
U.S. Pat. No. 3,574,672 discloses a process for depositing W.sub.2 C by heating a substrate to a temperature between 400.degree.-1300.degree. C. The process described in this patent is essentially the same as disclosed in U.S. Pat. No. 3,389,977.
U.S. Pat. No. 3,721,577 discloses a process for depositing refractory metal or metal carbides on ferrous and non-ferrous base materials heated to at least 1050.degree. C. The metal carbides are deposited using halide vapors of the metal along with methane and hydrogen.
U.S. Pat. No. 3,814,625 discloses a process for the formation of tungsten and molybdenum carbide by reacting a mixture of WF.sub.6 or MoF.sub.6, benzene, toluene or xylene and hydrogen. The process is carried out under atmospheric pressure and temperatures ranging from 400.degree.-1000.degree. C. An atomic ratio of W/C in the gaseous mixture varying from 1 to 2 is required to yield W.sub.2 C. The process also suggests that for some substrates such as mild steel, it is advantageous in providing better adhesion to deposit a layer of nickel or cobalt prior to tungsten carbide deposition. The process also claims the formation of a mixture of tungsten and tungsten carbide in the presence of large proportions of free hydrogen. The mixture of W and W.sub.2 C coating deposited according to the teaching of the '625 patent consists of columnar grains. The '625 patent does not disclose a process for depositing a tungsten carbide coating in non-columnar fashion.
British Patent 1,326,769 discloses a method for the formation of tungsten carbide by reacting a mixture of WF.sub.6, benzene, toluene or xylene and hydrogen under atmospheric pressure and temperatures ranging from 400.degree.-1000.degree. C. The process disclosed in this patent is essentially the same as disclosed in U.S. Pat. No. 3,814,625.
British Patent 1,540,718 discloses a process for the formation of W.sub.3 C using a mixture of WF.sub.6, benzene, toluene or xylene and hydrogen under sub-atmospheric pressure and temperature ranging from 350.degree.-500.degree. C. An atomic ratio of W/C in the gaseous mixture varying from 3-6 is required to yield W.sub.3 C. The coating deposited according to the teaching of British Patent 1,540,718 consists of columnar grains. The British '718 patent does not teach a process for depositing a non-columnar coating.
The methods of the Holzl patents cited above have been useful in producing fine-grained tungsten/carbon alloys containing mixtures of N and WC, and N and A15 structure, and the methods described in other cited patents have been successful in producing columnar W.sub.3 C or W.sub.2 C or mixtures of W and W.sub.2 C. The method of the presently claimed invention is one in which an extremely hard, fine-grained, non-columnar substantially pure tungsten carbide coating is deposited on a substrate. Prior art methods for depositing similar non-columnar coatings are disclosed and claimed in a number of recently issued patents.
U.S. Pat. No. 4,874,642, issued Oct. 17, 1989 to Diwakar Garg et al. discloses and claims coatings comprising non-columnar tungsten-carbide alloys containing mixtures of tungsten and true carbides in the form of W.sub.2 C or W.sub.3 C or a mixture of W.sub.2 C and W.sub.3 C. The disclosed tungsten/carbon alloys are free of columnar grains and consist essentially of extremely fine, equiaxial crystals. The deposits are produced at temperatures in the range of 300.degree. to 650.degree. C. Such alloys have been found to be especially useful since the presence of the W.sub.2 C and/or W.sub.3 C carbides in non-columnar microstructure contributes to both the hardness and the tensile strength of the deposited materials.
In U.S. Pat. No. 4,855,188, issued Aug. 8, 1989 to Diwakar Garg et al, filed Feb. 8, 1988 , a highly erosive and abrasive wear resistant composite coating system is described in which an intermediate layer of substantially pure tungsten Is deposited prior to depositing tungsten/carbon alloy coating. The outer tungsten/carbon alloy layer is comprised of a mixture of tungsten and tungsten carbide, with the tungsten carbide phase comprising of W.sub.2 C, W.sub.3 C or a mixture of both. The thickness of the intermediate tungsten layer is controlled in a manner to reduce or eliminate micro-cracks in the composite coating system and confer substantial erosive and abrasive wear characteristics on the composite coating system. The ratio of the thickness of the inner tungsten layer to the thickness of the outer tungsten/carbon alloy layer is at least above 0.3 in the cases Of W+W.sub.3 C, W+W.sub.2 C+W.sub.3 C and W+W.sub.2 C coatings. Additionally, the ratio of the thickness of the inner layer to the thickness of the outer layer to get optimum erosion and abrasion wear performance is at least 0.35 in the case of mixtures of tungsten and W.sub.2 C in the outer layer, 0.60 in the case of mixtures of tungsten and W.sub.3 C in the outer layer and, 0.35 in the case of mixtures of tungsten and W.sub.2 C and W.sub.3 C in the outer layer.
While such alloy coatings of the Garg et al patents have excellent wear resistance, the presently claimed coatings have significantly improved wear resistance over such prior art non-columnar coatings in both humid and lubricated environments.